Background: This observational study aims to describe fitness, and objectively measured physical activity levels and patterns in 409 young black South African adults (aged 19&ndash;20&nbsp;years) from Soweto, as well as to examine associations between physical activity, fitness and BMI.

Methods: A sub-maximal ramped step test was used to obtain an estimate of maximal oxygen uptake (VO2max). Physical activity was measured using ActiGraph (GT1M) for 7&nbsp;days in 256 participants. Time spent in sedentary (&lt;100 counts per minute (cpm)), moderate (2020&ndash;5998&nbsp;cpm) and vigorous (&ge;5999&nbsp;cpm) intensity activity was calculated, and 90% of participants were considered active. Data are presented as mean(CI) or median(CI).

Results: Overweight and obesity was more prevalent in females than males (35% vs 8%, p&nbsp;&lt;&nbsp;0.001). Males had a higher VO2max than females (41.9(41, 43) vs 32.6(32, 33)mlO2/kg/min, p&nbsp;&lt;&nbsp;0.001); spent more time in moderate to vigorous intensity physical activity (MVPA) (83(80, 94) vs 43(38, 45)min/day, p&nbsp;&lt;&nbsp;0.001), and less time in sedentary behaviours (541(541, 567) vs 575(568, 597)min/day, p&nbsp;&lt;&nbsp;0.01). Sedentary time was not associated with VO2max, however BMI was inversely associated, and MVPA was positively associated, with VO2max (both p&nbsp;&lt;&nbsp;0.001).

Conclusions: The majority of young South African adults in this study were sufficiently active, and higher MVPA was associated with fitness. However, the high level of sedentary behaviour in this population is of concern and may be contributing to the increasing prevalence of overweight and obesity in this population. Young South African females are at greatest risk for decreased cardiovascular fitness and should be the focus for future interventions.

Electronic supplementary material: The online version of this article (doi:10.1186/s12889-017-4212-0) contains supplementary material, which is available to authorized users.

Fig2: Distribution of estimated VO2max by BMI category for males and females

Mentions:
Estimated VO2max according to BMI category is presented in Fig. 2, for males and females separately. In both males and females, estimated VO2max was higher in underweight compared to overweight and obese participants (p < 0.05), and higher in normal weight compared to overweight and obese participants (p < 0.0001).Fig. 2

Fig2: Distribution of estimated VO2max by BMI category for males and females

Mentions:
Estimated VO2max according to BMI category is presented in Fig. 2, for males and females separately. In both males and females, estimated VO2max was higher in underweight compared to overweight and obese participants (p < 0.05), and higher in normal weight compared to overweight and obese participants (p < 0.0001).Fig. 2

Background: This observational study aims to describe fitness, and objectively measured physical activity levels and patterns in 409 young black South African adults (aged 19&ndash;20&nbsp;years) from Soweto, as well as to examine associations between physical activity, fitness and BMI.

Methods: A sub-maximal ramped step test was used to obtain an estimate of maximal oxygen uptake (VO2max). Physical activity was measured using ActiGraph (GT1M) for 7&nbsp;days in 256 participants. Time spent in sedentary (&lt;100 counts per minute (cpm)), moderate (2020&ndash;5998&nbsp;cpm) and vigorous (&ge;5999&nbsp;cpm) intensity activity was calculated, and 90% of participants were considered active. Data are presented as mean(CI) or median(CI).

Results: Overweight and obesity was more prevalent in females than males (35% vs 8%, p&nbsp;&lt;&nbsp;0.001). Males had a higher VO2max than females (41.9(41, 43) vs 32.6(32, 33)mlO2/kg/min, p&nbsp;&lt;&nbsp;0.001); spent more time in moderate to vigorous intensity physical activity (MVPA) (83(80, 94) vs 43(38, 45)min/day, p&nbsp;&lt;&nbsp;0.001), and less time in sedentary behaviours (541(541, 567) vs 575(568, 597)min/day, p&nbsp;&lt;&nbsp;0.01). Sedentary time was not associated with VO2max, however BMI was inversely associated, and MVPA was positively associated, with VO2max (both p&nbsp;&lt;&nbsp;0.001).

Conclusions: The majority of young South African adults in this study were sufficiently active, and higher MVPA was associated with fitness. However, the high level of sedentary behaviour in this population is of concern and may be contributing to the increasing prevalence of overweight and obesity in this population. Young South African females are at greatest risk for decreased cardiovascular fitness and should be the focus for future interventions.

Electronic supplementary material: The online version of this article (doi:10.1186/s12889-017-4212-0) contains supplementary material, which is available to authorized users.